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A growing body of evidence indicates that tree mycorrhizal association and species richness pronouncedly affect soil nitrogen (N) cycling. However, most studies have been conducted in temperate forests, and the roles of soil microbial communities have been largely overlooked. Here, based on 11 long-term dynamic monitoring sites in tropical forests, we investigated the effects of the dominance of trees associated with arbuscular mycorrhizal (AM) (by basal area) and tree species richness on soil N cycling, and explored the role of soil bacteria, archaea and fungi. Our results showed contrasting effects of AM tree dominance and tree species richness, with AM tree dominance-induced increases in soil ammonium N, nitrate N and the rate of net N nitrification, and tree species richness-caused decreases in ammonium N, nitrate N, the rate of net N mineralization and ammonification. Moreover, we found that the "plant-soil-microbe" interactions mainly triggered the changes in net N mineralization, ammonification and nitrification. It was mainly bacterial composition that directly affected soil N mineralization and ammonification, while fungal diversity only changed soil N ammonification. For soil N nitrification, the abundance of ammonia-oxidizing archaea (AOA) was the main drivers. These findings reveal the discrepancy in effects of the AM tree dominance and tree species richness on soil N cycling, highlighting microbe-mediated mechanisms in regulating soil N mineralization, ammonification and nitrification. • AM tree dominance and tree species richness oppositely affected soil N cycling. • AM tree dominance and tree species richness modulated soil microbiome. • Microbes were crucial in mediating tree effects on soil N cycling. [ABSTRACT FROM AUTHOR]